This invention relates generally to glass-ceramic cooktop appliances and more particularly to the long term calibration of a device for sensing properties relating to the appliance.
The use of glass-ceramic plates as cooktops in cooking appliances is well known. Such glass-ceramic cooktops have a smooth surface that presents a pleasing appearance and is easily cleaned in that the smooth, continuous surface prevents spillovers from falling onto the heating unit underneath the cooktop.
In one known type of glass-ceramic cooktop appliance, the glass-ceramic plate is heated by radiation from a heating unit, such as an electric coil or a gas burner, disposed beneath the plate. The glass-ceramic plate is sufficiently heated by the heating unit to heat utensils upon it primarily by conduction from the heated glass-ceramic plate to the utensil. Another type of glass-ceramic cooktop appliance uses a heating unit that radiates substantially in the infrared region in combination with a glass-ceramic plate that is substantially transparent to such radiation. In these appliances, a utensil placed on the cooktop is heated primarily by radiation transmitted directly from the heating unit to the utensil, rather than by conduction from the glass-ceramic plate. Such radiant glass-ceramic cooktops are more thermally efficient than other glass-ceramic cooktops and have the further advantage of responding more quickly to changes in the power level applied to the heating unit.
In both types of glass-ceramic cooktop appliances, provision must be made to avoid overheating the cooktop. For most glass-ceramic materials, the operating temperature should not exceed 700xc2x0 C. for any prolonged period. During operation, conditions can occur which can cause this temperature limit to be exceeded. Commonly occurring examples include operating the appliance with no load, i.e., no utensil, on the cooktop surface, using warped utensils that make uneven contact with the cooktop surface, and operating the appliance with a shiny and/or empty utensil.
To protect the glass-ceramic from extreme temperatures, glass-ceramic cooktop appliances ordinarily have some sort of temperature sensing device that can cause the heating unit to be shut down if high temperatures are detected. In addition to providing thermal protection, such temperature sensors can be used to provide temperature-based control of the cooking surface and to provide a hot surface indication, such as a warning light, after a burner has been turned off.
One common approach to sensing temperature in glass-ceramic cooktop appliances is to place a temperature sensor directly on the underside of the glass-ceramic plate. With this approach, however, the temperature sensor is subject to the high burner temperatures and thus more susceptible to failure. Moreover, direct contact sensors detect an average flux across the contact and do not produce a direct measurement of the glass-ceramic temperature. Thus, it is desirable to use an optical sensor assembly that xe2x80x9clooksxe2x80x9d at the glass-ceramic surface from a remote location to detect the temperature of the surface. Remote sensor assemblies are also capable of xe2x80x9clookingxe2x80x9d through the glass-ceramic plate to detect characteristics of a utensil placed on the cooktop, such as the temperature, size or type of the utensil, the presence or absence of the utensil, or the properties, such as boiling state, of the utensil contents.
Remote sensor assemblies are calibrated such that the sensor output signal will accurately represent the cooktop related property being detected. Over time, however, the system will experience certain effects that will affect the calibration and performance of the sensor assembly. These long term effects include aging of the glass-ceramic plate resulting in changes in its emissivity and reflectivity, formation of deposits on the glass-ceramic plate, the aging effects of the system""s optical components, and drifts and variations in system electronics.
Accordingly, there is a need for a remote sensor assembly that can monitor and compensate for long term changes.
The above-mentioned needs are met by the present invention which provides a sensor assembly for glass-ceramic cooktop appliances that includes an optical detector having a reference component and an active component. The active component is arranged to receive radiation from the glass-ceramic plate, and the reference component is insulated from radiation from the glass-ceramic plate. The sensor assembly further includes a temperature sensor located adjacent to the reference component, means for exciting the reference component, and a controller having a first input connected to the optical detector and a second input connected to the temperature sensor. The controller is responsive to the optical detector and the temperature sensor to calibrate the sensor assembly. Calibration is accomplished by noting the temperature reading of the temperature sensor after the burner assembly has not been used for a predetermined period of time to obtain a first calibration point. Then, the burner assembly is activated so that the temperature of the glass-ceramic plate is raised, and the output of the optical detector is noted. Next, the exciting means are used to heat the reference component. Alternatively, the reference component could be heated first, followed by heating the glass-ceramic plate. Either way, when the output of the optical detector reaches zero, the temperature reading of the temperature sensor is noted and used with the noted optical detector output to obtain a second calibration point. The first and second calibration points are used to calibrate the sensor assembly.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.